This invention relates to a gravity gradiometer for measurement of gravity gradient such as a change in the force of gravity experienced in a traversal over earth's terrain or experienced during passage past a massive object. More particularly, the invention relates to a gravity gradiometer having eight accelerometers disposed on a rotating disc and spaced uniformly around an axis of rotation of the disc to enable combination of measurement signals of the eight accelerometers with cancellation of signal components resulting from a frequency of rotation of the disc.
A gravity gradiometer measures difference in the intensity of a first gravitational field by moving an accelerometer via a circular path between two spaced-apart locations. For example, a gradiometer may be carried by a helicopter to measure features of the earth which affect the intensity and direction of the local gravitational field. Such features may include underground water and mineral deposits, as well as the presence of mountains.
In a typical construction of a gravity gradiometer, plural accelerometers have been carried by a disc rotating about a central axis and having a diameter of, for example, approximately 15 centimeters. An accelerometer is oriented with its input axis perpendicular to a radius of the disc. If the disc were oriented in a plane perpendicular to the surface of the earth, upon rotation of the disc, an accelerometer would pass through one location in the direction of the earth's gravitational field and through a second location 15 centimeters distant from the first location, in the direction opposite to the gravitational field. Upon comparison of the signals outputted by the accelerometer in the first and in the second locations, a difference is noted due to the change in direction of the field relative to the accelerometer input axis, and a further difference may be noted due to a possible change in the magnitude of gravitational field between the first and the second locations.
By using a matched pair of accelerometers disposed on opposite pins of a common diameter of the disc, the two signals of the two locations are provided simultaneously to facilitate their comparison. In addition, combination of the two signals provides for cancellation of any displacement of the apparatus of the gradiometer, including any displacement of a housing enclosing the disk and its attendant electrical circuitry.
The gradient T is measured in terms of difference of gravitational acceleration (g.about.980 cm/sec.sup.2) between the foregoing two locations, and the distance between the two locations. The gradient is given by the ratio of the difference of gravitational acceleration to the distance (in centimeters) between the two locations, and is given in terms of the unit Eotvos, wherein one Eotvos=10.sup.-9 sec.sup.-2. To obtain a complete description of the gravity gradient, in terms of a tensor, an assembly of three discs rotating about three mutually orthogonal axes is employed. Each disc has carried a set of four accelerometers as described above. However, the number of discs to be employed depends on the nature of the data required. For example, only a single disc rotating about a suitably chosen axis is required to detect a difference in gravitational field brought on by the presence of a massive object.
A problem arises in the situation wherein one tries to increase the rotational rate of the disc. Generally, a relatively slow rate of rotation, such as 15 rpm (revolutions per minute), is suitable for obtaining data as to the earth's terrain from a stationary or nearly stationary vehicle. However, if one were to obtain such data about a massive object, such as a railroad car, passing close to the gradiometer, by way of example, then a higher rotational rate, possibly 50 or 100 rpm, may be required. However, attempts to operate a gradiometer of the foregoing construction at the higher rotational rates has been unsuccessful because of additional signals appearing in the output of the gradiometer, the additional signals being a function of the rotational speed of the disc of the gradiometer. The additional signals tend to mask the desired output of the gradiometer and thereby diminish its utility.